New scratch build completed.

[bgrady]

Hey everybody, long time lurker here. Just finished this earlier today.
Here is a peak.

[img1041]http://i.imgur.com/FoJg8rU.jpg[/img]

The preamp is basically a couple of garden variety 12ax7 triode stages feeding a two knob EQ (similar to Big Muff tone control, but with a variable mid scoop), which drives a bootstrapped DC coupled CF. Standard sort of long tail pair PI, which drives a pair of scaled (plates + screens) cathode biased 6V6s. Front Panel is (from right) Volume, Bright Switch, Mids, Tone, Pre Pi master,
PPIMV, Voltage Scaling.

Like a modded plexi into a 5E3 output stage.

In any case, the amp sounds great for the most part, but cannot manage to overdrive the PI, even when the tone stack is bypassed! I cannot fathom this, as the PI (22K tail, 1k bias, 100k plates) should clip with maybe 10 V P-P, but does nothing when presented with 200V P-P!? I checked the voltages, and both the CF and PI have 300V on the plates, so it isn't that.

Can anyone can enlighten me as to why the PI might have so much headroom?

[tubeswell]

Lets see your schematic then.

[matt h]

(deleted)

[Leo_Gnardo]

Wow, what a beauty. I'm diggin' the finish on the chassis top. How you do that?

Stay tuned - we'll help sort out that gain/drive problem.

A look inside when you get a chance, too.

[bgrady]

The schematic is shown below for the CF and PI; the originals are actually hand drawn and virtually illegible. Note the tonestack is missing, as right now i is bypassed in an effort to supply the PI with as much input as possible.

[img:909:740]http://i.imgur.com/cnxTYu4.png[/img]

I may have used (really) loose terminology; there are three gain stages, with the last one being DC coupled to the CF. I think of them as a unit, so I got mixed up. The bootstrapping involves connecting the cathode of the CF to the mid point in V2s plate resistor (really two resistors), effectively making the resistor a constant source, but without affecting the available swing. This should up the gain of the previous stage to around mu, and indeed, there is buckets of usable, non fizzy gain.

I am thinking that the problem is the tail resistor;if the resistor there is larger, there should be much less voltage swing, right?

[mhartman]

That looks awesome!

[bgrady]

Here are the guts, just for the hell of it. I intend on cleaning up the wiring, but seeing as it doesn't seem to suffer from parasitic oscillations or noise, I am not too concerned.

[img1944]http://i.imgur.com/2o6VXlf.jpg[/img]

BTW the finish on the chassis is engine turning using a scour pad on a drill press. It turned out alright, but the chassis was actually painted before, and so has a rough surface from all the sanding.

[Jana]

The schematic doesn't look the same as the picture.

What's the deal with R11? It should be 100R.

What is that gray cap in the PI for?

A lot of the solder joints look questionable.

[bgrady]

The "grey cap" in the pi is a trim pot used to set the bias resistor to taste. I will be taking them out once I tune them properly.

Some parts of the amp construction do look sketchy, but I have traced everything, and there hasn't been anything to indicate problems. I will be redoing aspects of the wiring which were done in haste for an upcoming event.

It is exactly as per the schematic, but only the 3rd gain stage, CF, and PI are shown, not the first two gain stages! Input means the output of the second stage

The grid stopper for the CF is indeed 100k, as I literally just traced the amp. That appears to be a mistake. We might have found it!

[matt h]

(deleted)

[Jana]

Oh, and here I thought the schematic was the entire amp!

I suggest you take a little more patience with your builds. As I said, it doesn't look like the schematic matches the picture. I see a 4K7 resistor where you say there is a 47K on the schematic. I can't tell what the value of the other supposed 47K is (it is hidden under the orange drop).

I repeat, a lot of your solder joints look questionable.